During investigations of the system Sc2O3-Al2O3-TiO2-SiO2, a new, unusual microporous compound, Cs3ScSi8O19, was synthesized as colourless plates from a CsF-MoO3 flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo-Kα radiation, CCD area detector). The compound is orthorhombic, space group Pnma, with a = 11.286(2), b = 7.033(1), c = 26.714(5) Å, and Z = 4 (R1(F) = 2.6% and wR2all(F2) = 7.3%, using 3066 ‘observed’ reflections with Fo > 4σ(Fo)). The crystal structure of Cs3ScSi8O19 represents a new microporous framework structure type (‘MCV-1’), and the compound is exceptional in being the first representative of a mixed octahedral-tetrahedral framework structure, in which the [TO4]:[MO6] ratio is >6:1. The structure is based on isolated, nearly regular ScO6 octahedra [dav(Sc—O) = 2.112 Å] sharing corners with SiO4 tetrahedra to form an open framework with four-, six- and eight-membered rings; the latter are formed by SiO4 tetrahedra only. Two fully occupied Cs positions are located in large framework voids close to the six-membered rings, whereas four partly occupied and disordered Cs positions are close to very large framework voids bordered by the puckered eight-membered rings. The cavities are linked into channels parallel to [100] and [010]. The structure is compared with that of Cs2TiSi6O15 and related microporous scandium-, REE-, titano- and zirconosilicate minerals and compounds. Cs3ScSi8O19 or derivatives may be important in the context of immobilization of radioactive 137Cs waste, cationic conductivity or catalysis.